The purpose of a catalytic converter is to convert pollutant materials in engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides etc., to carbon dioxide, nitrogen, and water. Conventional catalytic converters utilize an oval cross-section ceramic honeycomb monolith 4 to 8 inches long having square, circular, triangular or hexagonal axially extending straight through openings or cells with a noble metal catalyst deposited in the cells. Other types of catalytic converters include catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads, and a corrugated thin metal foil monolith, e.g., ferritic stainless steel foil, having a catalyst supported on the surface, usually a refractory metal oxide surface. The catalyst is normally a noble metal metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more such metals. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby pollutant ingredients in the exhaust are converted to harmless by-products which then pass through the exhaust system to the atmosphere.
However, conversion is not efficient initially when the exhaust gases and the converter are relatively cold. To be effective at a high conversion rate, the catalyst and the surface of the converter must be at a minimum temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC) including unburned hydrocarbons, and 1000 F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary to contact relatively cold exhaust gases with hot catalyst to effect satisfactory conversion at engine start-up. Both compression ignited and spark ignited internal combustion engines have this need. Gas turbines also have this need.
To achieve initial heating of the catalyst prior to engine start-up, current practice provides an electrically heatable catalytic converter formed usually of a corrugated thin metal foil monolith which is connected a voltage source, e.g., a 12 volt to 108 volt automotive battery, and power is supplied, preferably before, during and after engine ignition to elevate and maintain the temperature of the catalyst to at least 650 F. plus or minus 20 F. The initial heat up time prior to engine ignition is from 2 to 30 seconds, and post crank heating, on demand.
The resistance of the corrugated thin metal monolith is used to heat the converter and accordingly electric power must be supplied to the monolith at opposite ends of a corrugated thin metal strip or strips from which the monolith is made. To accomplish this, at least one electrode must extend through the housing and be electrically isolated from the housing. Where only one electrode is used, the housing, being attached to the chassis, becomes the opposite pole of the voltage source. Where two electrodes of opposite charge are used, both must extend, in electrically isolated manner, through the housing and be attached to the monolith.
With prior insulated terminals or electrodes, gas leakage has occurred and it has now been found desirable to pneumatically seal the electrode or electrodes. Leakage is unacceptable because it causes oxygen sensors in the vehicle's emission system to malfunction.
Reference may be had to U.S. Pat. No. 4,711,009 to Cornelison et al dated Dec. 8, 1987 for details of a process for corrugating and coating thin metal foil strips and applying the catalyst, which process without the final steps of creasing and folding the strip may be used herein. Lengths of coated corrugated strip are secured as by welding to a tubular central core member, closed at least one end, and spirally wound about the core. The outer ends are brazed to an outer metallic shell.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this alloy will be found in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth selected from cerium, lanthanum, neodymium, yttrium, praseodymium, and mixtures of two or more thereof, balance iron, and trace steel making impurities.
In the following description, reference will also be made to fibrous ceramic mat or insulation. Reference may be had to the U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Sowman for formulation and manufacture of ceramic fibers and mats useful herein. One such ceramic fiber material is currently commercially available from 3-M under the registered trademark "INTERAM."
A brazing foil, which is an alloy of nickel, chromium, silicon, and boron useful herein is available commercially from Allied Metglas Products of Parsippany, N.J.